Calculator Inputs
Example Data Table
| Scenario | DC V | AC V | Load W | Surge W | Eff % | PF | Suggested Inverter | Battery Need |
|---|---|---|---|---|---|---|---|---|
| Small cabin backup | 24 | 230 | 600 | 1200 | 92 | 0.90 | 800 W | 143 Ah @ 24 V |
| Home essentials circuit | 12 | 120 | 400 | 800 | 90 | 0.85 | 800 W | 412 Ah @ 12 V |
| Workshop tools backup | 48 | 230 | 1800 | 3600 | 93 | 0.92 | 3000 W | 168 Ah @ 48 V |
These examples show typical sizing outcomes. Actual inverter selection should follow manufacturer surge duration, battery chemistry limits, and site wiring constraints.
Formula Used
Core inverter sizing formulas
- Adjusted continuous load = Continuous load × (1 + safety margin)
- Adjusted surge load = Surge load × (1 + safety margin)
- Required inverter watts = Adjusted continuous load ÷ derate factor
- Required inverter VA = Required inverter watts ÷ power factor
- DC input power = Adjusted continuous load ÷ inverter efficiency
Current and battery formulas
- Single-phase AC current = Watts ÷ (AC volts × power factor)
- Three-phase AC current = Watts ÷ (√3 × AC volts × power factor)
- DC current = DC input power ÷ DC voltage
- Backup energy = Load watts × backup hours
- Battery Ah needed = Backup Wh ÷ (efficiency × DoD × derate × DC volts)
These formulas provide planning estimates. Real systems also depend on ambient temperature, inverter surge duration, battery discharge curves, and wiring losses.
How to Use This Calculator
- Enter the battery bank or DC bus voltage for your solar setup.
- Input the AC output voltage needed by your appliances or subpanel.
- Add the continuous running load in watts and the highest startup surge.
- Enter expected daily energy use and required backup hours.
- Set inverter efficiency, power factor, derating allowance, and discharge limit.
- Choose the phase type and waveform preference.
- Press the calculate button to view inverter size, current, losses, and battery requirements.
- Use the CSV and PDF buttons to save the result summary.
Frequently Asked Questions
1. Why are both watts and VA shown?
Watts measure real power consumed by loads. VA measures apparent power that the inverter must support. Loads with lower power factor need more VA than watts, so both values matter when sizing safely.
2. Why should I include a safety margin?
A margin covers wiring losses, unknown future loads, hot-weather derating, and real operating conditions. It helps prevent nuisance shutdowns and keeps the inverter from running at its limit too often.
3. Why does lower DC voltage increase current so much?
For the same power, lower voltage means higher current. High current increases cable size, fuse ratings, heat, and voltage drop. That is why larger systems often move from 12V to 24V or 48V.
4. What does inverter efficiency change in the result?
Efficiency affects DC input power, battery demand, and heat loss. Lower efficiency means the inverter draws more energy from batteries and produces more heat for the same AC output.
5. Why is battery depth of discharge included?
Batteries are rarely designed for 100% usable capacity. Limiting discharge protects cycle life and usable voltage. A lower allowable discharge means you need a larger battery bank for the same backup time.
6. Is surge rating really that important?
Yes. Motors, pumps, fridges, compressors, and power tools often need much higher startup power than running power. An inverter that meets only continuous demand may still trip when equipment starts.
7. Which waveform should I choose?
Pure sine wave is best for most modern electronics, chargers, motors, and sensitive devices. Modified wave units may cost less, but they can create noise, heat, and compatibility problems.
8. Does this calculator replace final engineering review?
No. It is a planning tool for sizing estimates. Final selection should also consider manufacturer data, surge duration, cable sizing, breaker coordination, battery chemistry, ventilation, and local code rules.